Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver salt photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)”, published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)”, published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver salt photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
In such a recording method in dye diffusion transfer system, it has been known that it is important 1) to make the image-receiving sheet have high heat insulation and cushion properties, and 2) to use a receiving layer high in affinity for dyes, in order to give a favorable image (see, for example, the above “Information Recording (Hard Copy) and New Development of Recording Materials”, published by Toray Research Center Inc., 1993, pp. 241-285; and “Development of Printer Materials”, published by CMC Publishing Co., Ltd., 1995, p. 180).
Thus, in some cases, a composite support in which a biaxial oriented (stretched) polyolefin film containing microvoids was laminated with a core layer made, for example, of paper, is used as a base material for the image-receiving sheet, to make the image-receiving sheet have heat insulation and cushion properties (see, for example, U.S. Pat. No. 866,282, and JP-A-3-268998 (“JP-A” means unexamined published Japanese patent application)). However, this method involves drawbacks of lowering the productivity and increasing the production cost, since, in that method, a receptor layer is to be formed by solvent coating after the lamination process.
As other known methods of making the image-receiving sheet show heat insulation and cushion properties, a method in which, for example, a foaming layer composed of a resin and a foaming agent (see, e.g., Japanese Patent No. 2541796) or a porous layer containing hollow polymer particles (see, e.g., JP-A-2006-82382), each layer having high cushion properties, is formed between the support and the receptor layer, is known. Since, according to these methods, it is possible to form a heat-insulating layer on a base material by coating, the methods have such advantage that it is possible to omit the lamination process that is necessitated by the aforementioned method in which a composite support made of a biaxially-oriented polyolefin film containing microvoids is used.
In these techniques (Japanese Patent No. 2541796 and JP-A-2006-82382), however, although the heat-insulating layers are formed by aqueous coating, the receptor layers are formed by solvent coating. As polymers usable in the receptor layers because of their high affinity for dyes, polyester resins, vinyl chloride resins, and polycarbonate resins are known (see, e.g., JP-A-61-283595, JP-A-5-209118 and JP-A-6-227160). While solvent coating is adopted in many of the methods for forming those receptor layers, no method using a latex polymer permitting aqueous coating is carried out yet. Accordingly, the heat-sensitive transfer image-receiving sheet production inevitably entails a sequential coating process that first comes the formation of a heat-insulating layer by aqueous coating, and then the formation of a receptor layer by solvent coating, so it is hard to say that the methods as mentioned above are sufficient from the viewpoint of productivity.
In the silver-salt photographic industry, on the other hand, it is known that productivity is largely enhanced by adopting a simultaneous aqueous multilayer coating method in forming on a support a plurality of layers differing in their functions (JP-A-63-54975; and by Edgar B. Gutoff et al., “Coating and Drying Defects: Troubleshooting Operating Problems”, pages 101-103, John Wiley & Sons (1995)). Further, in the field of heat-sensitive transfer image-receiving sheets also, simultaneous multilayer coating has recently been put forth (JP-A-2006-88691). The aqueous multilayer coating has greater advantages than the solvent coating, in not only productivity, but also prevention of air pollution and hazard of fire, and improvement in working hygiene, but it has many problems to settle. Moreover, it is demanded for heat-sensitive transfer image-receiving sheets that they have much higher dyeing property than before, give sufficiently high sensitivity and maximum density, and exhibit good releasing property from ink sheets (no adhesion remains between those two sheets).